The Effect of Copper Vacancies on the Optical Bowing of Chalcopyrite Cu(In,Ga)Se 2 Alloys
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THE EFFECT OF COPPER VACANCIES ON THE OPTICAL BOWING OF CHALCOPYRITE Cu(In,Ga)Se 2 ALLOYS
D.S. ALBIN, J.J. CARAPELLA, J.R. TUTTLE, AND R. NOUFI Solar Energy Research Institute, 1617 Cole Blvd., Golden, CO 80401
ABSTRACT The optical bowing behavior of polycrystalline thin film Culni yGaySe2 alloys is dependent upon the Cu stoichiometry. The variation in optical band gap, Eg, for alloys in which Cu is near2 stoichiometric (25 at.%) is parabolic and follows the relationship: Eg(y) = 1.011 + 0.421 y + 0.244 y (eV), where y is the alloy parameter, [at.% Ga] / [at.%Ga + at.%In]. Contrary to this, films with Cu-poor stoichiometries (-19 at.% Cu) exhibit little alloy bowing: Eg(y) = 1.01 + 0.733 y - 0.046 y2 (eV). The increase in Eg with Cu deficiency appears to be the result of both a structural effect associated with tetragonal lattice shrinkage, AV = Vstoichiometric - VCu-poor (resulting from the presence of Cu vacancies) and a chemical effect, possibly associated with a counterbalance between p-d repulsion and anion displacement effects.
INTRODUCTION Recent interest in solar cells based on polycrystalline thin film CulnSe2 (CIS) has been stimulated by marked improvements in both device efficiency and manufacturability [1]. The development of large-area, cost-effective fabrication processes capable of producing high-efficiency small-area (14.1% at 3.5 cm 2 ) and large-area (9.7% at 0.4 m2 ) photovoltaic devices has been feasible [2,3,4]. Solar cells made of this material are typically of the "back wall" heterojunction design, in which the solar flux is transmitted through a wide-band-gap, n-type window material (ZnO/CdS) into a smallergap, p-type absorber (CIS). This layered structure is supported by a metal-coated (molybdenum) inexpensive substrate, typically soda-lime glass. Recent improvements in cell performance have been achieved by incorporating Ga within the active absorber layer. These modifications are shown in Figure 1 and include the use of Ga at the back surface [5] (reportedly less for alloying and more for improved film adhesion), uniformly throughout the bulk [6], and directly at the interface [7,8]. Improved device performance in the latter two approaches is a direct result of the increased band gap of the Culnl-yGaySe2 relative to CIS alone. CulnSe2 and CuGaSe2 belong to the family of tetrahedrally coordinated semiconductors known as AIBIIICV12 chalcopyrites [9,10]. Their tetragonal structure is derived from sphalerite (zinc blende) with the occurence of I-Ill ordering on the cation sublattice, which results in both an internal displacement of the anion (u*0.25) and an external tetragonal distortion (c/a * 2.0). Alloying these two compounds implies a statistical distribution of Ga with In while still maintaining the Cu-Ill order characteristic of chalcopyrite. Results of previous optical studies for this alloy system vary widely with respect to the degree of optical bowing present in the system. Single-crystal studies by Abid, et al. [11] and Bodnar, et al. [12] reported positive bowing parameters of
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